In a telecommunications network according to the above each operator belongs to a public land mobile network (PLMN), and a user's own operator is called a home public land mobile network (HPLMN). The user can visit other PLMNs and there are mechanisms to exchange charging data so that the user can be billed correctly. In UMTS (Universal Mobile Telecommunications System), the charging mechanisms have been specified by 3GPP (Third Generation Partnership Project). When a user switches from one PLMN to another PLMN, this is called roaming.
One operator typically operates on 2 or 3 frequency carriers. Each carrier will only support one operator. In every cell there can be several frequency carrier belonging to different operators.
In order for a mobile terminal, for example a mobile phone, to initially find a cell and a frequency carrier in WCDMA (wideband code-division multiple access, the access method in UMTS) an initial cell selection procedure is performed.
The procedure is described in “Introduction to 3G mobile communication” by Juha Korhonen and 3GPP doc 25.214 Annex C, and includes the following steps from the mobile phone's point of view:
Initial Cell-selection Procedure
Slot Synchronization
1) Search for primary synchronization channels (P-SCH). All P-SCHs have the same fixed primary synchronization code.
2) Once such a channel is found, acquire time slot synchronization from it. This is typically done with a single matched filter (or any similar device) matched to the primary synchronization code which, as mentioned above, is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output.
Frame Synchronization and Code-group Identification
3) Acquire frame synchronization from the corresponding secondary synchronization channels (S-SCH). This is done by correlating the received signal with all possible secondary synchronization code sequences, and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronization is determined.
Scrambling-code Detection
4) Acquire the primary scrambling code from the corresponding CPICH (common pilot channel).
5) Decode system information from the cell to check whether it is a suitable cell for camping (i.e. it contains the right PLMN code and access to it is allowed). The mobile phone determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. After the primary scrambling code has been identified, the Primary CCPCH (common control physical channel) can be detected, and the system and cell specific BCH (broadcast channel) information can be read. If the mobile phone has received information about which scrambling codes to search for, the procedure can be optimised.
The procedure is executed on one carrier frequency at a time. The mobile phone uses a frequency list which can be a full frequency list with all possible frequencies. The procedure is searching for a suitable frequency in the list. A problem with the frequency list is that the searching can be time consuming.